Fluoride ISE

EXAMPLE 5-3 Calculate the error in millivolts that would occur if a solution containing 5xlO-sMF- (pH 10) were measured with a fluoride ISE. 

To learn how the fluoride ISE works, and to understand how it should be used. 

The selectivity of the fluoride electrode is usually superb - perhaps the best for any of the solid-state ISEs. The reason why hydroxide ought to be avoided in tandem with a fluoride ISE is because the OH and F ions have a very similar ionic radius, and so move through the doped LaF3 lattice with a similar velocity and activation energy, i.e. the OH and F" ions are virtually indistinguishable by this method. 

We saw earlier in Section 3.5.2.2 how the selectivity of an ion-selective electrode (ISE) can be occasionally problematic, e.g. the activity of fluoride ion when determined by a fluoride ISE can be significantly in error if the solution also contains sufficient hydroxide ion to raise the pH above about 8. 

Figure 4.10 shows a Gran plot which can be used for such a situation. In this figure, the addition values on the right-hand side of the abscissa represent the volumes (i.e. the known additions) of contaminant brought about by adding, e.g. small amounts of hydroxide to a solution in which a fluoride ISE is immersed. The intercept of the plot on the left-hand side represents the equivalent amount of hydroxide within the sample prior to the multiple standard addition process. This intercept tells us the concentration of hydroxide. Therefore, knowing [OH ], we... 

It is incorrect to vacuum-plate the internal side of the membrane with silver or gold in the fluoride ISE containing the lanthanum trifluoride membrane [177]. 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

The solution pH affects the function of all ISEs, either through interference of hydroxonium or hydroxide ions in the membrane reaction (for example the interference of OH" with the function of fluoride ISEs at pH values greater than about 5.5), or through chemical interference in solution (for example, formation of poorly dissociated HF and HF in acid solutions, which are not sensed by a fluoride ISE or formation of HS and H2S with decreasing pH, which are not sensed by a sulphide ISE), or both. Moreover, the pH value can affect the equiUbria of interferents in the solution. The pH must thus be adjusted with all these effects in mind. Fortunately, it is usually sufficient to maintain the pH within a certain region rather than at a single precise value. 

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

The sensitivity of determination of traces of F can be increased by extraction with tetraphenylantimony(V)dichloride in CCI4 in the presence of CDTA and phosphate buffer. After back-extraction into an aqueous solution, the extractant is removed by shaking with CCI4. This concentrated solution is analyzed using a fluoride ISE and down to 10 m F can be determined in the original solution [55h]. For determination of F" after electrochemical generation of fluoride, see [88]. 

Fluoride ISEs have found important applications in the study of equilibria in which fluoride ions participate, especially in determination of the stability constants of fluoride coordination compounds [18, 29, 40,41, 53a, 71, 101,... 

The concentration of fluoride in water can usually be determined directly without pre-treatment. Among the numerous published analytical techniques, potentiome-try with fluoride ISE, ion chromatography, and spectrophotometry are commonly used. If the amount of fluoride present in water is very low, pre-concentration may be required. 

Total fluorine in fluoride supplements and dental products could be determined with minimal samples pre-treatment as for example by direct acid extraction or heating in TISAB buffer solution and subsequent determination of fluoride using fluoride ISE for the reason that entire fluorine, in these products, should be, by definition, available as free inorganic fluoride. 

A 0.200 g sample of toothpaste was suspended in SO ml of fluoride ionic-strength buffering medium (nSAB), and boiled briefly to extract the fluoride. The mixture was cooled, transferred quantitatively to a 100 ml volumetric flask, and diluted to volume with deionized water. A 25.00 ml aliquot was transferred to a beaker, a fluoride ISE and reference electrode inserted, and a potential of — 1SS.3 mV was obtained after equilibration. A 0.10 ml spike of O.S mg/ml fluoride stock solution was added after which the potential was — 176.2 mV. Calculate the percentage of F by weight in the original toothpaste sample. 

Figure 15.9 A commercial solid-state fluoride ISE. The ion-sensitive area is a solid crystal of Lap3 doped with Eup2. The filling solution contains NaP and NaCl. The electrical connection is made by an Ag/AgCl electrode. [Courtesy of Thermo Orion, Beverly, MA (www.thermoorion.com).]...

The fluoride ISE is used routinely for measuring fluoridated water and fluoride ion in dental products such as mouthwash. A 50 mL aliquot of water containing sodium fluoride is analyzed using a fluoride ion electrode and the method of standard additions. The pH and ionic strength are adjusted so that all fluoride ion is present as free F ion. The potential of the ISE/reference electrode combination in a 50 mL aliquot of the water was —0.1805 V. Addition of 0.5 mL of a 100 mg/L F ion standard solution to the beaker changed the potential to —0.3490 V. Calculate the concentration of (1) fluoride ion and (2) sodium fluoride in the water sample. 

Almost all flowing-stream methods reported to date for free and total fluoride determination and applied to quality control and environmental monitoring are related to potentiometric detection using the fluoride ISE owing to its high selectivity. Spec-trophotometric alternatives based either on the Alizarin complexone methodology or on the inhibitory effect of fluoride on the formation of different metal chelates have also accomplished a great approval. 

The fluoride ISE is very popular, because it is highly selective and reliable, the determination of... 

The fluoride ISE has been the most successful of all ISEs. This is so partly because it has uniquely good characteristics including a long service lifetime and partly because the potentiometric fluoride determination is so much more convenient than any... 

Hydrogen fluoride is determined either by a gas probe (Table 2), or, more often, by a fluoride ISE (lanthanum fluoride based) after absorption in a suitable medium. Hydrogen fluoride emissions can be measured down to a content of 0.1 pgm . ... 

Addition of TiSAB prior to fluoride-ISE determination Addition of TiSAB... 

The fluoride ISE is the most commonly used single-crystal ISE. It is shown schematically in Figure 15.9. The membrane is a single aystal of LaFj doped with EuFj. The term doped means that a small amount of another substance (in this case, EuFj) has been added intentionally into the LaFj crystal. (If the addition were not intentional, we would call the europium an impurity or contaminant )... 

Well-established potentiometric devices used in bioanalysis are based on macroscopic electrodes that use gas-permeable polymer, solid or liquid membranes selective to CO2, NH3, pH, or various alkali, alkaline earth, or halide ions [7]. The membranes separate the external analyte solution from the inner reference solution, and these electrodes are used in conjunction with an integral or external reference electrode. The selectivities of these membranes can be excellent, for example, the main interferent in the fluoride ISE that uses a solid Lap3/Eup2 membrane is hydroxide, with a Kfjoh value of 0.1 thus, over the pH range 0-8.5, the electrode responds in a Nernstian fashion to fluoride concentrations above 10 M [9]. Solid Ag2S membranes doped with AgX, where X is Cl , Br , I , CN , or SCN , are used for the determination of these anions. 

If a cell is constructed with a fluoride ISE combined with a SCE, calibration with solutions of fluoride ions of known concentration enable the value of k to be found. When the same cell is filled with an unknown solution, the fluoride ion concentration may be determined. This is an example of direct potentiometry. 

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